1. Field of the Invention
The present invention relates to a light source providing a wavelength-selective output. In more detail, it relates to a wavelength-tunable light source whose output wavelength can be externally controlled and a wavelength-division multiplexed (WDM) transmission system using the source.
2. Description of the Related Art
The light source providing output at a specific wavelength is one of the key-elements of a wavelength-division multiplexed transmission system, in which each channel is discriminated by its wavelength.
In order to minimize the interference between adjacent channels, the light source of wavelength-division multiplexed transmission system should have a stable wavelength and a sufficient side mode suppression ratio (SMSR). It is also desirable to have a high output power and a narrow line width.
A distributed feed-back laser diode (DFBLD) is a representative light source in the prior art that meets the requirements described above.
However, the DFBLD is very expensive and requires a complicated control process to fix its output wavelength at a specified point. A spectrum-sliced system, which uses a broad-band light source instead of wavelength-specified light source, has been demonstrated to reduce the cost and the complexity of system.
Incoherent broad-band light sources (ILSs) such as a light emitting diode (LED), a super-luminescent diode (SLD), and optical amplifiers emitting amplified spontaneous emissions (ASEs) are representative light sources being used for spectrum-sliced system. Spectrum-sliced systems using these light sources are very attractive since they are able to simply the wavelength control process compared with the distributed feed-back laser diode.
U.S. Pat. No. 5,440,417 (System for spectrum-sliced fiber amplifier light for multi-channel wavelength-division-multiplexed applications) discloses a method of spectrum-slicing by using optical amplifier light source. And U.S. Pat. No. 5,694,234 (Wavelength division multiplexing passive optical network including broadcast overlay) discloses a spectrum-sliced system by using a directly-modulated LED.
However, they also have several disadvantages. For example, LED or an SLD hardly provides sufficient output power and an optical amplifier light source requires an expensive external modulator even though its output power is comparatively large.
That is to say, the system presented in U.S. Pat. No. 5,440,417 requires an additional external modulator and the system presented in U.S. Pat. No. 5,694,234 hardly provides sufficient output power.
On the other hand, a wavelength-tunable light source improves the functionality of a wavelength-division multiplexed transmission system.
One can tune the output wavelength of a distributed feed-back laser diode (DFBLD) by temperature control, however, the tunable wavelength range is only about a few nanometers in 1270˜1600 nm band, the low-loss wavelength regime of a general silica-based single mode fiber.
Therefore, wavelength-tunable light sources employing an external cavity have been mainly studied in the prior art, however, they are costly and require complex devices to tune the output wavelength.